Means for supporting fuel elements in a nuclear reactor

ABSTRACT

5. A grid structure for a nuclear reactor fuel assembly comprising a plurality of connecting members forming .[.at least one.]. longitudinally extending .[.opening.]. .Iadd.peripheral and inner fuel element openings .Iaddend.through .Iadd.each of .Iaddend.which .Iadd.openings .Iaddend. at least one nuclear fuel element extends, said connecting members forming wall means surrounding .[.said.]. .Iadd.each peripheral and inner fuel element .Iaddend.opening, a pair of rigid projections .Badd.longitudinally spaced from one another .Baddend.extending from a portion of said wall means into .[.said.]. .Iadd.each peripheral and inner .Iaddend.opening for rigidly engaging .[.said.]. .Iadd.each .Iaddend.fuel element, .Iadd.respectively, yet permit individual longitudinal slippage thereof, .Iaddend.and resilient means .Badd.formed integrally on and from said wall means and .Baddend.positioned in .[.said.]. .Iadd.each peripheral and inner .Iaddend.opening in opposed relationship with said projections and located to engage said fuel element to bias the latter into engagement with said rigid projections, .Iadd.respectively. .Iaddend.

The present invention relates to nuclear reactors and more particularlyto means for supporting elements bearing fissionable material or fuel ina nuclear reactor core, but the invention can also be used to supportelements in any fluid media.

One general structural form commonly used for providing a nuclear fuelinventory in nuclear reactors is that in which numerous elongatedelements or rods containing fissile material are arranged, within aprescribed volume, in a parallel array in an upstanding directionbetween upper and lower reactor core support plates. To provideintegrity in the support relations, the fuel rods are divided intogroups and the rods in each group are formed as a fuel assembly prior toplacement between the reactor core support plates. A fluid, havingcoolant and, if desired, neutron moderating properties, flows along andamong the fuel rods as a vehicle for energy transfer.

Generally, means are normally provided for laterally supporting the fuelelements for reasons including that of resisting lateral displacement soas to prevent localized neutron flux peaking with resulting "hot spots"or regions of extreme temperature rise in the fuel elements. Therefore,if the "hot spots" can be eliminated, the reactor can be operated closerto its design power limit.

In the past a spring finger type grid design has been utilized tosupport laterally the fuel rods in a fuel assembly. This design conceptprovides for individual support of each fuel rod at axially spacedintervals by spring fingers, which position the fuel rods radiallywithin the fuel assembly while permitting thermal expansion with aminimum of constraint. Although the spring finger grid design has provento be an effective and economical means of providing fuel rod support inthe past fuel assembly designs, the problems associated with this designwhich pose design restrictions are fretting wear, the number of gridsrequired to provide adequate fuel rod support, and the care requiredduring assembly and shipping of the fuel assemblies to prevent damage tothe grid spring fingers.

A significant fact contributing to the aforementioned problems is therelatively small resistance to fuel rod lateral motion inherent incurrent grid designs which, in principle, behave as an unstable springand mass system because the fuel rods are supported laterally by springfingers alone. Any forces tending to cause motion of the fuel rod arenot resisted (neglecting friction) until deflection of the springfingers is such as to build up a force to stop the motion. Therefore,before any large resisting force is produced, motion has alreadycommenced. This motion even though it may be only microscopic inmagnitude may cause serious fretting under the loads and bearingstresses involved at reactor operating conditions for extended periodsof time. Also, the number of grids required to provide adequate fuel rodsupport has an adverse effect on hydraulic and nuclear core designparameters. Another problem encountered is the extreme care andprecautions against shock and accelerations during handling and shippingwhich are required to prevent damage to the grid spring fingers.

Accordingly, it is the general object of this invention to provide anovel means for supporting fuel elements in a fuel assembly.

Another object of this invention is to provide means for laterallysupporting the fuel elements in a fuel assembly while enabling the fuelelements to respond to applied thermal or other forces with axialmovement relative to the lateral supporting means.

Still another object of this invention is to provide a grid member whichcreates a greater resistance to fuel rod motion.

A further object of this invention is to provide an improved grid designwhich permits a greater unsupported length of fuel rods resulting infewer grids and material in the fuel assembly.

Another object of this invention is to provide a grid member withgreater structural integrity and greater resistance against damage fromshock and acceleration forces encountered in normal handling andshipping.

Yet another object of this invention is to provide a grid member havinga more stable and rugged spring finger and backup tab design, so thateach fuel rod passing through the grid member is forced to assume alocation in the center of the grid cell and is not permitted to assumean off-center position obtained by deflecting one spring finger morethan another.

Briefly, the present invention accomplishes the above cited objects byproviding a more stable spring and mass system as well as a built-inbeam effect resulting in a resisting moment. The aforementioned resultis accomplished by having two relatively rigid projections engaging thefuel element on one side and a spring means engaging the fuel elementdirectly opposite to the rigid projections. Another set of identicallateral supports are provided for the elongated fuel element at the samelongitudinal locations of the fuel element, but rotated approximately90°. Therefore, a six point lateral support is provided for the fuelelement.

More specifically, a nuclear fuel arrangement or assembly comprises aplurality of elongated parallel fuel elements which are supportedrelative to each other through the use of supporting means includingelongated frame means. At least one grid member extends laterally acrossthe frame means and has respective openings through which the fuelelements extend. The openings in the grid members are formed by aplurality of straps, which are interwoven to provide a structuralnetwork, similar to an "egg crate." The grid straps, in turn, areprovided with rigid and resilient means projecting into each opening forsupporting the fuel elements against lateral displacement and, to agiven extent, frictionally against longitudinal movement. The rigidmeans comprises two longitudinally spaced rigid projections which engagethe fuel element on one side. The resilient means comprises a springwhich engages the fuel element directly opposite to the rigidprojections at a longitudinal point preferably midway between the rigidprojections. Another identical set of lateral supports are formed in theother two straps which form the opening so as to support the fuelelement at the same longitudinal location, but at a lateral locationapproximately 90° apart from the first set of lateral supports. Thus, asix point lateral support is provided for each fuel element at eachopening in the grid member.

Further objects, features and advantages of the invention will becomeapparent as the following description proceeds wherein features ofnovelty, which characterize the invention, will be pointed out withparticularity.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIG. 1 is a partially sectioned elevational view of a fuel assemblyformed in accordance with the principles of this invention and is takenalong reference line I--I of FIG. 2;

FIG. 2 is an enlarged cross sectional view of the fuel assembly of FIG.1 and is taken along reference line II--II of FIG. 1 to show the supportrelationship between a grid member and the various fuel elements;

FIG. 3 is a longitudinally sectioned view of a fuel assembly and istaken along reference line III--III of FIG. 2 to show the rigid andresilient lateral supports in one of the grid straps;

FIG. 4 is a longitudinally sectioned view of a portion of a grid strapand is taken along reference line IV--IV of FIG. 3 to show the supportrelationship between a grid strap and the various fuel elements;

FIG. 5 is an isometric view of a plurality of grid straps with one strapbeing removed from the grid structure so as to show how the straps maybe interlaced into a grid-like structure of "egg crate";

FIG. 6 is an enlarged longitudinally sectioned view of a grid strap ofFIG. 5 and is taken along the reference line VI--VI of FIG. 5 to showanother species of a rigid lateral support portion of the rigid andresilient combination used to support a fuel element;

FIG. 7 is a cross sectional view of a fuel assembly taken just above thegrid structure so as to show another arrangement of the rigid andresilient lateral supports wherein resilient lateral supports arelocated around the periphery of the grid structure;

FIG. 8A is a longitudinally sectioned view of a portion of the gridstructure of FIG. 7 and is taken along reference line VIII--VIII to showthe combination of rigid lateral supporting means at certain locationsin the grid structure so as to make it possible to have only resilientlateral supporting means around the periphery of the grid structure;

FIG. 8B is another embodiment of FIG. 8A, which serves the same purposeand can be used in lieu of the arrangement shown in FIG. 8A;

FIG. 9 is an elevational view of a portion of a grid structure showingan arrangement where only laterally resilient means are used to supporta fuel element;

FIG. 10 is an equilibrium diagram with no externally applied forcesacting on a fuel element in the grid arrangement of FIG. 9;

FIG. 11 is an equilibrium diagram with externally applied forces;tending to cause motion on a fuel element in the grid arrangement ofFIG. 9;

FIG. 12 is an equilibrium diagram with no externally applied forcesacting on a fuel element in the grid arrangement of FIG. 6;

FIG. 13 is an equilibrium diagram with externally applied forces actingon a fuel element in the grid arrangement of FIG. 6; and

FIG. 14 is a view in elevation of a portion of a grid structure of thisinvention showing a different fuel element arrangement therein.

Referring now to FIG. 1 there is shown a fuel assembly 20 comprising aplurality of elongated parallel fuel elements 22, which suitably containa given quantity of fissionable material, and supporting means for thefuel elements 22 including elongated frame means 24 in which the fuelelements 22 are located. In this instance, the frame means or member 24includes a plurality of arms 26, an upper end plate 28, an enclosure orcan 30, and a lower end plate 32. The arms 26 have a recess 34 thereinfor engagement with a remote handling tool (not shown). The upper endplate 28 is secured to the bottom of the arms 26 and has flow openings36 and fuel element openings 38. The enclosure 30, in turn, is securedto the upper end plate 28 and the lower end plate 32. The fuel elements22 merely rest on the lower end plate 32; and, therefore, the fuelelements 22, which extend through the fuel element openings 38, can beremoved individually with the fuel assembly 20 remaining in place in thereactor core (not shown). Flow openings 40 are provided in the can 30 soas to permit cross flow of the employed coolant, and the form of thecoolant openings 40 is such as to maximize both this flow and structuralstiffness. The lower end plate 32 also has flow openings (not shown) isa direction axially parallel to the fuel elements so as to permitcoolant flow through the fuel assembly 20.

The material selected for use in forming the frame member 24 shouldprovide maximum structural strength consistent with low neutronabsorption cross section. For example, stainless steel is suitable forthis purpose.

A grid member or structure 42 is secured to and extended laterallyacross the can 30 for the purpose of providing lateral support for thefuel elements of rods 22. A plurality of grids 42 can be secured to thecan 30 at various spacings along the length of the can 30 so as toprovide the amount of lateral support desired.

As viewed in FIGS. 2, 3 and 4, the grid 42 is comprised of a pluralityof inner straps 44 which are interwoven in a grid-like manner.Generally, this configuration is similar to that described in copendingapplication Ser. No. 326,070, entitled "Fuel Arrangement for a NuclearReactor," filed Nov. 26, 1963, now abandoned by Erling Frisch, and alsoassigned to the present assignee, which is a continuation of copendingapplication Ser. No. 19,851, entitled "Fuel Arrangement for a NuclearReactor," now abandoned, filed Apr. 4, 1960, by Erling Frisch and alsoassigned to the present assignee. Therefore, the specific description ofthe grid 42 will be limited here to the subject matter which pertains tothe present invention. Thus, it is to be noted that the inner straps 44are interfitted through the use of opposing latching means or slots (asindicated by the reference character 46 in FIG. 5) which allow thestraps 44 to be interlatched at their various intersections.

In the formation of the grid 42, the latching means are employed withall of the inner straps 44 as well as the boundary straps 45. When thestraps 44 and 45 are preassembled as described, the entire assembly isthen permanently secured together, for example through the use of afurnace brazing operation or by welding. The grids 42 can be welded orotherwise secured to the can 30 so as to be supported suitably forperformance of the lateral fuel rod supporting function. In turn, thegrid or grids 42 also contribute to rigidizing the can 30.

The grid 42 is provided with respective openings 50 for .[.receipt ofthe fuel rods 22..]. .Iadd.In the example of FIG. 2, the grid 42comprises a plurality of peripheral fuel element openings and one innerfuel element opening disposed inwardly of the peripheral fuel elementopenings. However, as evident from the Frisch patent applicationheretofore referred to, which has matured into Patent 3,379,618, morethan one inner fuel element opening may be provided in the grid members42. .Iaddend.Each inner strap 44 is provided with resilient means, suchas spring fingers or strips 52, which are deflected from the strap 44into an adjacent opening 50 for lateral support of the fuel rods 22.Each inner strap 44 is also provided with rigid means, such asprojections or dimples 54, which are located above and below each springfinger 52 and are deflected from the strap plane in a direction oppositeto their respective spring fingers 52 so as to project into adjacentopenings 50 for rigid lateral support of the fuel rods 22. FIG. 3clearly shows a spring finger 52 with a dimple 54 both above and belowthe spring finger 52. FIG. 4, in turn, clearly shows that the springfinger 52 projects from the strap plane in a direction directly oppositeto the dimples 54.

Referring to FIG. 2, the dimples 54 are shown as projecting into eachopening 50 from the two adjacent inner straps 44. Spring fingers 52 canalso be seen projecting into the same opening from the other twoadjacent straps 44 which form the enclosure for the aforementionedopening. Therefore, each spring finger 52 engages the fuel rod 22 at alocation opposite from a pair of dimples 54. Returning to FIG. 3 fourguide dimples 56 and 58 are shown therein. The guide dimples 56 arealigned horizontally and project from the strap plane in one direction,while the guide dimples 58 are also aligned in a horizontal plane butproject from the strap plane in the opposite direction from the dimples56. The guide dimples 56 and 58 are tangent to the width of the slot 46and are located vertically opposite to the slot 46. The guide dimples 56and 58 serve to guide the straps 44 and 45 when the aforementionedstraps are inserted into each other.

FIG. 4 shows how the fuel element 22 is laterally supported in oneplane. Dimples 54b substantially rigidly support fuel rod 22b at points60 and 62, while spring finger 52b resiliently supports the fuel rod 22bat point 64. Thus, the fuel rod 22b is supported at three points in oneplane. An identical three-point lateral support is also provided in aplane perpendicular to the plane shown in FIG. 4. FIG. 4 also shows thatthe spring finger 52b engages one fuel rod 22b on one side of the strap44a while the dimples 54a on the same strap 44a engage another fuelelement 22a on the other side of the inner strap 44a. Also shown in FIG.4 is the fact that the point 64 where spring finger 52b engages the fuelrod 22b is located approximately midway between the two points 60 and 62where the two dimples 54b engage the fuel rod 22b.

Returning now to FIG. 2, only one type of lateral supporting means isrequired at the boundary straps 45, because openings 50 are only locatedon one side of the boundary straps 45. Therefore, only a pair of dimples54 or a spring finger 52 project into a single opening 50. Therefore,the arrangement is such that a spring finger 52 on boundary straps 45 islocated opposite to a pair of dimples 54 on the inner straps 44, or apair of dimples 54 on boundary straps 45 are located opposite to asingle spring finger 52 on inner straps 44.

All of the above features described in connection with the grid 42 aremore readily discernible in the isometric view shown in FIG. 5. However,in FIGS. 5 and 6, another species of a substantially rigid lateralsupporting means is shown. In this instance, the rigid lateralsupporting means is a slotted projection 66 which is formed from a strap44' and also 45'. Also shown in FIGS. 5 and 6 are several curves 68 and70 formed in the spring finger 52. The aforementioned curves have beenformed in the spring finger 52 so as to cause the spring finger 52 toflex at its ends and also to obtain the desired spring properties.

Referring now to FIG. 7, there is shown a grid structure which containsonly spring fingers 52 in the boundary straps 45". The grid 42", in thisinstance, is secured to four corner rods .Iadd.or spacers .Iaddend.72.The rods 72, in turn, are secured to upper and lower end plates 28 and32, respectively, shown in FIG. 1. In this instance, the rods 72 replacethe can 30 previously described in connection with FIG. 1. In this typeof a canless fuel assembly, it is advantageous to have only springfingers 52 around the periphery, because if a boundary strap 45" is bentduring handling, there will still be resilience available from thespring finger 52. On the other hand, if a rigid lateral supportingmeans, such as slotted projections 66 or dimples 54, were located in theboundary straps 45", inward bending of boundary straps 45" would causethe laterally rigid supporting means to push the fuel rods 22 out ofplace.

In the configuration shown in FIG. 7, a double set of dimples must beused at a single vertical location within certain adjacent grid openings50. In FIG. 8A there is shown that dimples 54a protrude in one directionfrom the grid strap 44" so as to engage fuel element 22a, and anotherpair of dimples 54b protrude in the opposite direction from the plane ofthe grid strap 44" so as to engage fuel element 22b. FIG. 8B shows adifferent arrangement from FIG. 8A in that the two adjacent dimples 54dprotrude in one direction, whereas the two outer dimples 54c protrude inthe opposite direction.

In FIG. 14, the grid arrangement is formed with resilient means 52 andrigid means 54a formed only in some of the grid openings with the fuelelements 22a extending through only predetermined ones of the openings.

FIG. 9 shows a portion of a grid arrangement as described in theaforementioned copending application. In this configuration the fuel rod22 is laterally supported by four cantilever spring tabs located 90°apart from one another in a horizontal plane. FIG. 10 represents anequilibrium diagram with the forces shown in a single plane, whereinsymbol 76 represents the spring tab 74 acting on fuel element 22. Thespring tab 74 also has a spring constant K. Therefore, FIG. 10 shows thestatic forces applied to fuel element 22.

FIG. 11 shows an equilibrium diagram resulting from a displacing forcewith motion of the fuel element 22 impending. FIGS. 9 to 11 represent anunstable spring and mass system and no resisting moment.

FIGS. 6, 12 and 13, on the other hand, are the counterparts of FIGS. 9,10 and 11, respectively; however, FIGS.6, 12 and 13 represent the novelarrangement of this invention which utilizes a stable spring and masssystem with a built-in beam effect.

A comparison will now be made between the novel arrangement shown inFIG. 6 in contrast to a prior arrangement as shown in FIG. 9. In FIGS.10 to 13, the meaning of the symbols are as follows:

P=Preload on the fuel element 22 by the spring tab 74 or the springfinger 52

K=Spring constant

X=Amount of fuel element motion caused by a displacing force

M_(r) =Resisting moment to motion X

M_(d) =Displacing moment

F_(r) =Resisting force to motion X

F_(d) =Displacing force

D=Distance between the projections 66

Returning now to the old arrangements shown in FIGS. 9 to 11 and withthe application of external forces producing lateral fuel elementmotion:

    F.sub.4 =(P+KX)-(P-KX=2KX

(see FIG. 11)

M_(r) =F_(r) (0) as no moment exists between the opposing forces P (seeFIG. 10)

M_(r) =0

M_(d) is greater than 0, because of the application of external forces

Therefore, the old arrangement represents an unstable spring and masssystem since motion must commence before a resisting force F_(r)(neglecting friction) can be produced to react to the displacing forceF_(d). In addition, there is no resisting moment M_(r) produced within asingle grid structure 42 to counteract any displacing moment M_(d).

Returning now to the novel arrangement shown in FIGS. 6, 12 and 13 andwith the application of external forces producing lateral fuel elementmotion:

    F.sub.r =P+KX

(see FIG. 13) ##EQU1## as a moment arm exists between the point ofrotation (see FIG. 13) ##EQU2##

The improvement in the new arrangement of FIG. 6 is readilyascertainable when the magnitude of the forces resisting motion areconsidered. Specifically, the force resisting fuel element displacementin the old arrangement of FIG. 9 is 2KX as compared with P+KX in the newarrangement of FIG. 6. Furthermore, the resisting force in the oldarrangement of FIG. 9 requires X motion (neglecting friction) beforereaching equilibrium and offers no resisting moment. This is due to thefact that one cantilever spring tab was working against the otherthereby providing a resisting force which became dependent upon themotion of the spring tabs 74. This vibratory motion, even thoughmicroscopic, may result in wear and fretting of the fuel element 22.

The new arrangement of FIG. 6, however, produces a resisting force equalto the preload P on the spring finger 52; and the new arrangement alsoproduces an appreciable resisting moment, which first must be overcomebefore fuel element motion can occur. Therefore, this arrangementprovides the capability of establishing the spring preload forces andthe resulting resisting moment large enough in magnitude, in comparisonwith the mechanically and hydraulically induced displacing forcesexperienced in the operation of a reactor core, such that there will beno motion of the fuel elements 22 at the grids 42. Therefore, the newarrangement of FIG. 6 provides a more rigid support for the fuelelements 22 in the fuel assembly 20.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, modifications theretowill readily occur to those skilled in the art. For example, thecantilever spring tabs shown in FIG. 9 can be used to replace the springfingers in the preferred embodiment. In fact, the only thing that isrequired is a resilient lateral supporting means of some type. Also asolid piece of material can be secured to the strap to provide alaterally rigid supporting means in lieu of the dimples or slottedprojections. Another means for making the fuel assembly more rigid is torotate the grid member laterally 90° to 180° from an adjacent gridmember. The grid structure can also be made of a material such asInconel or stainless steel.

It is not desired, therefore, that the invention be limited to thespecific arrangements shown and described. However, it is intended tocover in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

We claim: .Badd..[..[.1. A grid structure for a nuclear reactor fuelassembly comprising a plurality of connecting members forming.[.respective openings.]. .Iadd.peripheral and inner fuel elementopenings .Iaddend.through .[.which.]. .Iadd.each of which openings an.Iaddend.elongated .[.elements are.]. .Iadd.fuel elements is.Iaddend.adapted to extend, at least two rigid spaced projectionsextending into each of .[.at least some of.]. said .[.grid.]..Iadd.peripheral and inner fuel element .Iaddend.openings from at leastone of said members, and at least one resilient means extending intoeach of said .[.some.]. .Iadd.peripheral and inner .Iaddend.openingsfrom at least another of said members and disposed generally opposite tosaid two rigid projections with its innermost protrusion beingpositioned in alignment with the region intermediate said spacedprojections..]..]. .Badd..[..[.2. The combination of claim 1 whereinonly one of said resilient means and said rigid means is located at theouter periphery of the grid structure..]..]..Baddend. .Badd..[..[.3. Thecombination of claim 1 wherein said projections of each of said.[.some.]. openings are longitudinally aligned, and wherein at leastsome of the innermost protrusions are longitudinally positioned midwaybetween the generally opposed rigid projections..]..]. .Badd..[..[.4. Agrid structure for a nuclear reactor fuel assembly comprising aplurality of connecting members forming .[.at least one opening.]..Iadd.peripheral and inner fuel element openings .Iaddend.through.Iadd.each of .Iaddend.which, .Iadd.respectively.Iaddend., at least oneelongated .Iadd.fuel .Iaddend.element extends and .[.forming.]..Iadd.forms .Iaddend.a segment of a flow channel .[.therebetween.]..Iadd.between said fuel elements.Iaddend., rigid means engaging one sideof .Iadd.all of .Iaddend.said elongated .[.element.]. .Iadd.elements,respectively.Iaddend., to laterally position .[.said.]. .Iadd.each.Iaddend.elongated element .Iadd.yet permit longitudinal slippagethereof.Iaddend., resilient means disposed on another side of .[.said.]..Iadd.each .Iaddend.elongated element generally opposite to said oneside and engaging .[.said.]. .Iadd.respectively each .Iaddend.elongatedelement to bias the latter into engagement with said rigid means, one ofsaid rigid means and said resilient means extending from said connectingmembers into said .[.openening,.]. .Iadd.peripheral and inner openings.Iaddend.and the other of said rigid means and said resilient meansbeing located in said flow channel..]..].
 5. A grid structure for anuclear reactor fuel assembly comprising a plurality of connectingmembers forming .[.at least one.]. longitudinally extending.[.opening.]. .Iadd.peripheral and inner fuel element openings.Iaddend.through .Iadd.each of .Iaddend.which .Iadd.openings .Iaddend.atleast one nuclear fuel element extends, said connecting members formingwall means surrounding .[.said.]. .Iadd.each peripheral and inner fuelelement .Iaddend.opening, a pair of rigid projections.Badd.longitudinally spaced from one another .Baddend.extending from aportion of said wall means into .[.said.]. .Iadd.each peripheral andinner .Iaddend.opening for rigidly engaging .[.said.]. .Iadd.each.Iaddend.fuel element, .Iadd.respectively, yet permit individuallongitudinal slippage thereof.Iaddend., and resilient means .Badd.formedintegrally on and from said wall means and .Baddend.positioned in.[.said.]. .Iadd.each peripheral and inner .Iaddend.opening in opposedrelationship with said projections and located to engage said fuelelement to bias the latter into engagement with said rigid projections,.Iadd.respectively.Iaddend..
 6. The combination of claim 5 wherein saidpair of .Badd.longitudinally spaced .Baddend.rigid projections arelongitudinally aligned on said wall means portion .[..[.and arelongitudinally spaced from one another.]..]., and wherein said resilientmeans is positioned in alignment with the region intermediate saidspaced projections. .Badd..[..[.7. The combination of claim 6 includinganother pair of longitudinally aligned spaced rigid projectionsextending from another wall means portion .Iadd.into each opening,respectively.Iaddend., said other wall means portion positionedlaterally with respect to said first wall portion .Iadd.of eachopening.Iaddend., and other resilient means positioned in .[.said.]..Iadd.each .Iaddend.opening in opposed relationship with said other pairof projections and positioned to engage said fuel element to bias thelatter into engagement with said other pair of projections..]..]..Badd..[..[.8. The combination of claim 5 wherein said resilient meansare formed integrally with said wall means..]..]..Baddend.
 9. A gridstructure for a nuclear reactor fuel assembly comprising a plurality ofinterlaced straps forming at least a pair of adjacent longitudinallyextending openings with each of said openings having at least oneelongated nuclear fuel element extending therethrough, at least a pairof longitudinally aligned spaced rigid projections extending from saidstraps into each of said openings for rigidly engaging the respectivefuel element, resilient means extending from said straps into each ofsaid openings in opposed relationship with said projections andpositioned to engage said respective fuel element to bias the latterinto engagement with the respective pair of rigid projections one ofsaid straps forming a common wall between said adjacent openings, one ofsaid pairs of spaced longitudinally aligned rigid projections beingformed on said common wall and extending into one of said openings, andone of said resilient means being formed on said common wall andpositioned intermediate and in longitudinal alignment with said.[..[.oen.]..]. .Badd.one .Baddend.pair of spaced projections, said oneresilient means extending into the other of said openings.
 10. A gridstructure for a nuclear reactor fuel assembly comprising a plurality ofinterlaced straps forming at least a pair of adjacent longitudinallyextending openings with each of said openings having at least oneelongated nuclear fuel element extending therethrough, at least a pairof longitudinally aligned spaced rigid projections extending from saidstraps into each of said openings for rigidly engaging the respectivefuel element, resilient means extending from said straps into each ofsaid openings in opposed relationship with said projections andpositioned to engage said respective fuel element to bias the latterinto engagement with the respective pair of rigid projections, one ofsaid straps forming a common wall between said adjacent openings, one ofsaid pairs of spaced longitudinally aligned rigid projections beingformed on said common wall and extending into one of said openings, andthe other of said pairs of spaced longitudinally aligned rigidprojections formed on said common wall and extending into the other ofsaid openings, said one and said other pairs of rigid projections beinglongitudinally aligned with one another.
 11. .[.The combination of claim4 wherein.]. .Iadd.A grid structure for a nuclear reactor fuel assemblycomprising a plurality of connecting members forming at least oneopening through which at least one elongated element extends and forminga segment of a flow channel therebetween, rigid means engaging one sideof said elongated element to laterally position said elongated element,resilient means disposed on another side of said elongated elementgenerally opposite to said one side and engaging said elongated elementto bias the latter into engagement with said rigid means, one of saidrigid means and said resilient means extending from said connectingmembers into said opening, and the other of said rigid means and saidresilient means being located in said flow channel, .Iaddend.saidconnecting members .[.are.]. .Iadd.being .Iaddend.formed from strapsextending laterally of said fuel elements, and including latching meansin said straps for interfitting said straps with each other, and guidemeans disposed on said straps longitudinally opposite to said latchingmeans for guiding the straps into an interfitted relationship.
 12. Thecombination of claim 11 wherein said latching means are a plurality ofslots extending partially through said straps in a longitudinaldirection, and wherein said guide means comprise a plurality of dimplesgrouped in pairs, and the dimples in each pair are laterally alignedwith each other and are spaced from each other approximately thethickness of said strap.
 13. A grid structure for a nuclear reactor fuelassembly comprising a plurality of connecting members formed frominterfitting cross-laced straps with .[.at least a portion.]..Iadd.portions .Iaddend.of four of said straps forming .[.one.].longitudinally extending .[.opening.]. .Iadd.peripheral and inner fuelelement openings .Iaddend.through .Iadd.each of .Iaddend.which at leastone nuclear fuel element extends, said connecting members forming wallmeans surrounding .[.said.]. .Iadd.each peripheral and inner fuelelement .Iaddend.opening, a pair of spaced longitudinally aligned rigidprojections extending from one of said strap portions into .[.said.]..Iadd.each fuel element .Iaddend.opening for rigidly engaging said fuelelement, first resilient means positioned in .[.said.]. .Iadd.each fuelelement .Iaddend.opening in opposed relationship with said projectionsand positioned in alignment with the region intermediate saidprojections to engage said fuel element to bias the latter intoengagement with said rigid projections another pair of longitudinallyaligned spaced rigid projections extending from another of said strapportions .Iadd.into each fuel element opening.Iaddend., said other strapportion positioned adacent to said first strap portion, and secondresilient means positioned in .[.said.]. .Iadd.each fuel element.Iaddend.opening in opposed relationship with said other pair ofprojections and positioned in alignment with the region intermediate theprojections of said other pair to engage .[.said.]. .Iadd.each.Iaddend.fuel element .Iadd.respectively .Iaddend.to bias the latterinto engagement with said other pair of projections, and said first andsaid second resilient means being formed respectively on .Badd.and from.Baddend.the remaining ones of said four strap portions.
 14. A fuelarrangement for a nuclear reactor comprising a plurality of elongatedfuel elements disposed in a generally parallel array, .[.at least onemember.]. .Iadd.a plurality of longitudinally spaced grid members.Iaddend.extending laterally across said fuel arrangement, .[.and.]..Iadd.said array including fuel elements that are permitted toindividually expand and contract longitudinally relative to said gridmembers, said grid members positioning laterally in spaced relationshipall of said fuel elements in said array that are permitted toindividually expand and contract longitudinally relative to said gridmembers, each of said grid members .Iaddend.having respective openings.[.through which.]. .Iadd.therein all of .Iaddend.said.Iadd.longitudinally expandable .Iaddend.fuel elements .[.extend.]..Iadd.in said array extending through respective openings in each gridmember, each of .Iaddend.said grid .[.member.]. .Iadd.members.Iaddend..Badd.formed by a plurality of rigid walls and .Baddend.havingboth resilient means and substantially rigid means extending into eachof .[.at least some of.]. said .Iadd.last mentioned openings forsupporting all of .Iaddend.said .Iadd.longitudinally expandable.Iaddend.fuel elements against lateral displacement, .Badd.said rigidmeans in each opening including at least two longitudinally spacedcontact regions integrally formed on a rigid wall, .Baddend.and saidresilient means being .Badd.longitudinally spaced from and.Baddend.disposed generally opposite to said rigid means .Badd.andformed integrally on and from said walls.Baddend..
 15. .[.A fuelarrangement for a nuclear reactor comprising a plurality of elongatedfuel elements disposed in a generally parallel array, at least one gridmember forming respective openings through which said fuel elementsextend, said grid member having resilient means and substantially rigidmeans resilient means and substantially extending into each of at leastsome of said openings for supporting said fuel elements against lateraldisplacement,.]. .Iadd.The fuel arrangement of claim 14 wherein.Iaddend.said rigid means .[.including.]. .Badd.forming said contactregions .Baddend..Iadd.includes .Iaddend.respective spaced projectionsextending into each of said .[.some.]. grid openings but being locatedopposite said resilient means, and said resilient means engaging saidfuel elements at a location between said projections. .Badd..[..[.16.The combination of claim 15 wherein said spaced projections are disposedin groups of at least two longitudinally alignedprojections..]..]..Baddend.
 17. A fuel arrangement for a nuclear reactorcomprising a plurality of elongated fuel elements disposed in agenerally parallel array, elongated frame means juxtaposed and parallelto said fuel elements, .[.at least one.]. .Iadd.a plurality oflongitudinally spaced .Iaddend.grid .[.member.]. .Iadd.members.Iaddend.being .[.secure.]. .Iadd.secured .Iaddend.to and extendinglaterally .[.across.]. .Iadd.from .Iaddend.said frame means .[.and.].,.Iadd.said array including fuel elements that are permitted toindivdually expand and contract longitudinally relative to said gridmembers, said grid members positioning laterally in spaced relationshipall of said fuel elements that are permitted to individually expand andcontract longitudinally and slip relative to said grid members, each ofsaid grid members .Iaddend.forming respective .Iadd.aligned.Iaddend.openings through at least some of which .Iadd.aligned openingseach of .Iaddend.said .Iadd.expanding .Iaddend.fuel elements.Iadd.respectively .Iaddend.extend, .Iadd.there being a group oflongitudinally aligned fuel element openings for each expanding fuelelement.Iaddend., said grid .[.member.]. .Iadd.members.Iaddend..Badd.being formed by a plurality of rigid walls and.Baddend.having resilient means and substantially rigid means extendinginto each of said fuel element openings for supporting .Iadd.all of.Iaddend.said .Iadd.expanding .Iaddend.fuel elements against lateraldisplacement, said resilient means including respective spring fingers.Badd.integrally formed on and from said walls and .Baddend.engagingsaid .Iadd.expanding .Iaddend.fuel elements, said rigid means includingrespective spaced projections extending .Badd.from said walls.Baddend.into each of said fuel element openings in groups of at leasttwo axially aligned projections but being located generally opposite ofsaid spring fingers, said spring fingers engaging said fuel elements ata location between said spaced projections, and four of said projectionsand two of said spring fingers extending into each fuel element opening.18. .[.A fuel arrangement for a nuclear reactor comprising a pluralityof elongated fuel elements disposed in a generally parallel array, atleast one grid member extending laterally across said fuel arrangementand having at least one opening through which at least one of said fuelelements extend, substantially rigid means for rigidly engaging andlaterally positioning said one fuel element, resilient means for biasingsaid one fuel element into engagement with said rigid means, saidresilient means being disposed generally opposite to said rigid means,and.]. .Iadd.The fuel arrangement of claim 14 wherein .Iaddend.at leastone of said rigid means and said resilient means .[.being.]. .Iadd.is.Iaddend.disposed on .Iadd.each of .Iaddend.said grid .[.member.]..Iadd.members .Iaddend.and .[.extending.]. .Iadd.extend .Iaddend.intosaid .[.opening.]. .Iadd.openings.Iaddend..
 19. A fuel arrangement for anuclear reactor comprising .[.at least one.]. .Iadd.a plurality of.Iaddend.elongated fuel .[.element.]. .Iadd.elements disposed in agenerally parallel array, means for positioning said fuel elements, saidarray including fuel elements that are permitted to individually expandand contract longtiduinally relative to said positioning means, saidmeans positioning laterally in spaced relationship all of said fuelelements that are permitted to individually expand and contractlongitudinally and slip relative to said positioning means, saidpositioning means engaging each of said expanding fuel elements and.Iaddend..Badd.formed by a plurality of rigid walls.Baddend..Iadd.including substantially .Iaddend.rigid means .Badd.onsaid walls .Baddend.engaging one side of .Iadd.each of .Iaddend.said.Iadd.expanding .Iaddend.fuel .[.element.]. .Iadd.elements .Iaddend.tolaterally position said .Iadd.expanding .Iaddend.fuel .[.element.]..Iadd.element.Iaddend., resilient means .Badd.integrally formed on andfrom said walls .Baddend.disposed on a side of .Iadd.each of.Iaddend.said .Iadd.expanding .Iaddend.fuel .[.element.]. .Iadd.elements.Iaddend.generally opposite to said one side and engaging said.Iadd.expanding .Iaddend.fuel .[.element.]. .Iadd.elements respectively.Iaddend.to bias the latter into engagement with said rigid means, oneof .Iadd.each of .Iaddend.said resilient means and said rigid meansbeing longitudinally spaced along said fuel .[.element.]. .Iadd.elementsrespectively .Iaddend.to form a pair of spaced members, and the other of.Iadd.each of .Iaddend.said resilient means and said rigid means beingpositioned intermediate said pair of members. .Iadd.
 20. The fuelarrangement of claim 19 including means for fixedly positioning each ofsaid rigid and resilient means longitudinally with respect to each ofsaid expanding fuel elements. .Iaddend..Iadd.
 21. The fuel arrangementof claim 20 including means for fixedly positioning said rigid meanslaterally with respect to each of said expanding fuel elements..Iaddend. .Iadd..[..[.22. A fuel arrangement for a nuclear reactorcomprising a plurality of elongated fuel elements disposed in agenerally parallel array, a plurality of longitudinally spaced gridmembers extending laterally across said fuel arrangement for positioninglaterally in spaced relationship all of said fuel elements in said arraythat are permitted to individually expand longitudinally and sliprelative to said grid members, said expanding fuel elements each of saidgrid members comprising a plurality of connecting members forming aplurality of fuel element openings, each of said expanding fuel elementsextending through an opening in each grid member, and forming a segmentof a flow channel between said fuel elements, substantially rigid meansrigidly engaging one side of each of said expanding fuel elementsrespectively to laterally position each expanding fuel element yetpermit longitudinal slippage thereof, resilient means disposed onanother side of each expanding fuel element generally opposite to saidone side and engaging respectively each expanding fuel element to biasthe latter into engagement with said rigid means, one of said rigidmeans and said resilient means extending from said connecting membersinto said opening, and the other of said rigid means and said resilientmeans being located in said flow channel..]..]..Iaddend..Iadd.
 23. Thefuel arrangement of claim 14 including rigid spacer means extendinglongitudinally between said grid members and fixedly secured thereto tospace said grid members longitudinally with respect to said expandingfuel elements. .Iaddend. .Iadd..[..[.24. The fuel arrangement of claim22 including rigid spacer means extending longitudinally between saidgrid members and fixedly secured thereto to space said grid memberslongitudinally with respect to said expanding fuelelements..]..]..Iaddend..Iadd.
 25. The fuel arrangement of claim 14wherein said resilient means is preloaded with a force sufficient toovercome those displacement forces respectively acting on said expandingfuel elements and tending to move said fuel elements laterally away fromsaid rigid means. .Iaddend..Iadd.
 26. .[..[.The fuel arrangement of 14wherein are.]..]..Iaddend. .Badd.A fuel arrangement for a nuclearreactor comprising a plurality of elongated fuel elements disposed in agenerally parallel array, a plurality of longitudinally spaced gridmembers extending laterally across said fuel arrangement, said arrayincluding fuel elements that are permitted to individually expand andcontract longitudinally relative to said grid members, said grid memberspositioning laterally in spaced relationship all of said fuel elementsin said array that are permitted to individually expand and contractlongitudinally relative to said grid members, each of said grid membershaving respective openings therein, all of said longitudinallyexpandable fuel elements in said array extending through respectiveopenings in each grid member, each of said grid members having bothresilient means and substantially rigid means extending into each ofsaid last mentioned openings for supporting all of said longitudinallyexpandable fuel elements against lateral displacement, and saidresilient means being disposed generally opposite to said rigid means,and .Baddend..Iadd.said resilient means formed integrally upon.Iaddend..Badd.and from .Baddend..Iadd.said grid members.Iaddend...Iadd.
 27. The fuel arrangement of claim 14 wherein each of said gridmembers is formed by a plurality of cross-laced interconnected rigidstraps. .Iaddend..Iadd.
 28. The fuel arrangement of claim 14 whereinsaid some fuel elements comprise at least a majority of said fuelelements in said fuel arrangement..Iaddend..Iadd.
 29. A fuel arrangementfor a nuclear reactor comprising a plurality of fuel elements disposedin a generally parallel array, means for supporting said fuel elementsfor limited longitudinal movement to permit expansion and contraction ofeach movable fuel element relative to said supporting means andindependently of expansion and contraction of the other longitudinallymovable fuel elements, a plurality of grid members spaced apartlongitudinally of said array and each extending laterally across saidarray for positioning laterally in spaced relationship all of saidlongitudinally movable fuel elements, rigid spacer means extendinglongitudinally of said array between said grid members and rigidlysecured to all of said grid members to space said grid memberslongitudinally with respect to the fuel elements, each of said gridmembers comprising peripheral straps and two sets of intersecting andrigidly interconnected inner straps with said inner straps of one setextending transversely of said inner straps of the other set, said innerand peripheral straps cooperating to form a plurality of openings,respective openings in said grids being aligned longitudinally of saidarray to form groups of aligned fuel element openings respectivelyaccommodating the longitudinally movable fuel elements passingtherethrough, there being a group of longitudinally aligned fuel elementopenings for each longitudinally movable fuel element, each said innerstrap including a plurality of pairs of rigid means formed integrallythereon, the rigid means of each pair being spaced from each otherlongitudinally of the array and the different pairs being spaced apartalong each strap in a direction transversely of the array, each pair ofsaid rigid means engaging one side of one of said longitudinally movablefuel elements to laterally position each such fuel element yet permitlongitudinal slippage thereof, a plurality of spaced apart resilientsprings formed integrally with each inner strap, each such resilientspring being formed longitudinally intermediate one pair of said rigidmeans and extending outwardly from said inner strap in a direction awayfrom the latter pair of rigid means, the resilient springs being spacedalong each inner strap in a direction transversely of the array and by adistance approximating the spacing between the pairs of rigid meansformed along that inner strap, said resilient springs engaging anotherside of said longitudinally movable fuel elements generally opposite tosaid one side to bias each of said longitudinally movable fuel elementsinto engagement with one pair of said rigid means, there being at leastone pair of rigid means and one spring extending into each fuel elementopening for the longitudinally movable fuel elements. .Iaddend..Iadd.30. The fuel arrangement of claim 29 wherein said resilient springs arepreloaded with a force sufficient to overcome those displacement forcesrespectively acting on said longitudinally movable fuel elements andtending to move said fuel elements laterally away from said rigid means..Iaddend.